jscript: Fix parsing async functions and methods

http://www.ecma-international.org/ecma-262/8.0/#sec-functions-and-classes

Fixes #1537.

Units/parser-javascript.r/js-async1.d/expected.tags

@@ -0,0 +1,3 @@
+add1	input.js	/^async function add1(x) {$/;"	f
+add2	input.js	/^async function add2(x) {$/;"	f
+resolveAfter2Seconds	input.js	/^function resolveAfter2Seconds(x) {$/;"	f

Units/parser-javascript.r/js-async1.d/input.js

@@ -0,0 +1,29 @@
+/* https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function#Simple_example */
+
+function resolveAfter2Seconds(x) {
+  return new Promise(resolve => {
+    setTimeout(() => {
+      resolve(x);
+    }, 2000);
+  });
+}
+
+async function add1(x) {
+  var a = resolveAfter2Seconds(20);
+  var b = resolveAfter2Seconds(30);
+  return x + await a + await b;
+}
+
+add1(10).then(v => {
+  console.log(v);  // prints 60 after 2 seconds.
+});
+
+async function add2(x) {
+  var a = await resolveAfter2Seconds(20);
+  var b = await resolveAfter2Seconds(30);
+  return x + a + b;
+}
+
+add2(10).then(v => {
+  console.log(v);  // prints 60 after 4 seconds.
+});

Units/parser-javascript.r/js-async2.d/expected.tags

@@ -0,0 +1,9 @@
+Class	input.js	/^var Class = {$/;"	c
+ES6Class	input.js	/^class ES6Class {$/;"	c
+asyncAnonymousFunc	input.js	/^var asyncAnonymousFunc = async function() {$/;"	f
+es6AsyncMethod	input.js	/^  async es6AsyncMethod() {}$/;"	m	class:ES6Class
+es6Method	input.js	/^  es6Method() {}$/;"	m	class:ES6Class
+method1	input.js	/^  method1 : function() {$/;"	m	class:Class
+method2	input.js	/^  method2 : async function() {$/;"	m	class:Class
+method3	input.js	/^  method3() {$/;"	m	class:Class
+method4	input.js	/^  async method4() {$/;"	m	class:Class

Units/parser-javascript.r/js-async2.d/input.js

@@ -0,0 +1,21 @@
+var Class = {
+  method1 : function() {
+    return 0;
+  },
+  method2 : async function() {
+    return await resolveAfter2Seconds(42);
+  },
+  // ES6 shorthand methods
+  method3() {
+  },
+  async method4() {
+  },
+};
+
+var asyncAnonymousFunc = async function() {
+}
+
+class ES6Class {
+  async es6AsyncMethod() {}
+  es6Method() {}
+}

parsers/jscript.c

@@ -130,6 +130,7 @@ enum eKeywordId {
 	KEYWORD_static,
 	KEYWORD_default,
 	KEYWORD_export,
+	KEYWORD_async,
 };
 typedef int keywordId; /* to allow KEYWORD_NONE */
 
@@ -226,8 +227,9 @@ static const keywordTable JsKeywordTable [] = {
 	{ "class",		KEYWORD_class				},
 	{ "extends",	KEYWORD_extends				},
 	{ "static",		KEYWORD_static				},
-	{ "default",		KEYWORD_default				},
+	{ "default",	KEYWORD_default				},
 	{ "export",		KEYWORD_export				},
+	{ "async",		KEYWORD_async				},
 };
 
 /*
@@ -1293,6 +1295,9 @@ static bool parseMethods (tokenInfo *const token, const tokenInfo *const class,
 			goto cleanUp;
 		}
 
+		if (isKeyword (token, KEYWORD_async))
+			readToken (token);
+
 		if (! isType (token, TOKEN_KEYWORD) &&
 		    ! isType (token, TOKEN_SEMICOLON))
 		{
@@ -1312,7 +1317,11 @@ static bool parseMethods (tokenInfo *const token, const tokenInfo *const class,
 			if ( isType (token, TOKEN_COLON) || is_shorthand )
 			{
 				if (! is_shorthand)
+				{
 					readToken (token);
+					if (isKeyword (token, KEYWORD_async))
+						readToken (token);
+				}
 				if ( is_shorthand || isKeyword (token, KEYWORD_function) )
 				{
 					JSCRIPT_DEBUG_PRINT("Seems to be a function or shorthand");
@@ -1778,6 +1787,9 @@ static bool parseStatement (tokenInfo *const token, tokenInfo *const parent, boo
 			readToken (token);
 		}
 
+		if (isKeyword (token, KEYWORD_async))
+			readToken (token);
+
 		if ( isKeyword (token, KEYWORD_function) )
 		{
 			vString *const signature = vStringNew ();
@@ -2143,6 +2155,7 @@ static bool parseLine (tokenInfo *const token, tokenInfo *const parent, bool is_
 				parseSwitch (token);
 				break;
 			case KEYWORD_return:
+			case KEYWORD_async:
 				readToken (token);
 				is_terminated = parseLine (token, parent, is_inside_class);
 				break;